Conventional alkaline electrochemical cells are formed of a cylindrical housing having an open end. The housing is initially formed with an enlarged open end. After the cell contents are supplied, the cell is closed by crimping the housing edge over an edge of the end cap assembly and radially compressing the housing around the assembly to provide a tight seal. The end cap assembly comprises an exposed terminal end cap plate and typically a plastic insulating member which forms a plug at the open end of the housing and insulates the terminal end cap plate from the cell housing. A problem associated with design of alkaline cells is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally around the point of complete exhaustion of the cell's useful capacity. Alkaline cell are conventionally provided with a rupturable member such as a diaphragm or membrane within the end cap assembly, for example, as described in U.S. Pat. No. 3,617,386. Such diaphragms or membranes are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured. The end cap assembly disclosed in U.S. Pat. No. 3,617,386 has considerable free space above the rupturable member, which reduces the amount of available space within the cell for active material.
In order to provide a tight seal alkaline cells have end cap assemblies which include a metal support disk inserted in a cavity within the plastic insulating member. The metal support disk may have a convoluted surface as shown in U.S. Pat. Nos. 5,532,081 or 5,080,985 which assures that end cap assembly can withstand high radial compressive forces during crimping of the cell's housing around the end cap assembly. Such support disk allows high radial forces to be applied during crimping. This results in a tight mechanical seal around the end cap assembly at all times. To provide additional support during crimping the peripheral edge of the terminal end cap may also be located in a cavity within the plastic insulating member as shown in U.S. Pat. No. 5,080,985. Such designs, however, can occupy additional space within the cell and can markedly increase the complexity of fabrication, particularly if applied to very small cells.
U.S. Pat. No. 4,670,362 discloses a plastic insulating disk which is snap fitted into the open end of a cylindrical casing for an alkaline cell. The disclosed insulating disk does not comprise nylon. The insulating disk disclosed in this reference is not contemplated for use in very small size cells, e.g., AAAA alkaline cells. The disclosed snap fitted insulating disk requires additional force for insertion into the open end of a cylindrical casing. Any additional force required for insertion of the insulating disk becomes a disadvantage in sealing very small diameter cells, e.g., AAAA size alkaline cells, because of the difficulty in handling such smaller cells.
A rupturable vent membrane may be integrally formed as part of the plastic insulating member included within the end cap assembly. Such vent membrane may typically be of circular shape as shown in U.S. Pat. No. 4,537,841. As shown in this reference the rupturable membrane may be integrally formed as a thin portion of the plastic insulating member. The rupturable membrane may also take the form of a grooved or circumferential configuration as disclosed in U.S. Pat. No. 5,080,985.
It is more difficult to obtain the required radial compression of the cell housing at the open end of the housing in sealing AAAA size cell than in sealing larger cells, without the use of a metal support disk or end cap plate captured within the housing. In part this is because in very small diameter alkaline cells, e.g. AAAA size cells, it becomes more difficult to step out the open end of housing out, i.e., enlarge the open end, when the housing is initially formed.
Another problem associated with the design of the end cap assemblies for AAAA alkaline cells is that any plastic insulating disk employed to seal the cell is of necessity very small diameter corresponding to the small cell diameter. Because such disks are very small, they may be more prone to cracking than plastic insulating disks for larger size cells.